As the number of services provided by wireless mobile communication devices increases dramatically, so does the need for mobile communication devices that can handle the various forms of signal formats required to provide these services. For example, devices in cellular telephones may need to adhere to standards such as a Global Systems for Mobile communication (GSM) standard, a Personal Communication Services (PCS) standard, an EDGE standard, and a Digital Cellular System (DCS) standard. The standards all require precise output power control over a large dynamic range in order to prevent channel interference.
The key component common to mobile communication devices is a power amplification device. Before reaching the power amplification device, a radio frequency (RF) transmission signal is too weak for transmission to a cellular base station. Therefore, it is the function of the power amplification device to boost the power of the RF transmission signal.
The power amplification device may receive the RF transmission signal with a constant envelope when the RF transmission signal is being transmitted in accordance with modern Time Division Multiple Access (TDMA) standards, such as GSM standards and PCS standards. After amplification by the amplification device, the RF transmission signal must comply with a specification known as a “burst mask.” The burst mask specifies the mean power of the RF transmission signal transmitted in a particular timeslot. More specifically, the burst mask specifies an allowable ramp-up period, duration, and ramp-down period of the mean power of the RF transmission signal during a timeslot. In a TDMA standard, there may be various and multiple timeslots each having a burst mask specification. The RF transmissions signal must conform to the various burst masks specifications for the different timeslots. If the power amplification device ramps up too slowly, data at the beginning of the burst might be loss, degrading link quality. On the other hand, if the power amplification device ramps up power too quickly, this has the effects of spreading the energy of the RF transmission signal across the spectrum therefore causing spectrum interference.
Generally, power amplification devices include voltage regulation circuits, such as low-drop-out (LDO) circuits, to provide a regulated voltage to a power amplification circuit that amplifies the RF transmission signal. The LDO circuit generates the regulated voltage from a supply voltage and regulates the regulated voltage level so that fluctuation in the supply voltage level of the supply voltage do not significantly affect the regulated voltage level. This regulated voltage determines the amplification gain of the power amplification circuit. For optimum rated efficiency, the power amplification circuit is driven to operate in saturation by the LDO circuit when the RF transmission signal is a TDMA transmission signal with a constant envelope. However, the LDO circuit should not be driven into saturation because saturation results in significant spectrum interference and a degraded switching spectrum. In essence, the power amplification circuit transitions from the linear region to the saturated region or from the saturated region to the linear region too quickly when the LDO circuit is driven into saturation.
To prevent the LDO circuit from operating in saturation, prior art designs of LDO circuits have been implemented in which the voltage adjustment gain of the LDO circuit is reduced when the regulated voltage level of the regulated voltage reaches of threshold voltage level. Unfortunately, prior art designs also detect when the regulated voltage level reaches the threshold voltage level relative to an arbitrarily set voltage level. While the threshold voltage level may be set near the saturation voltage level of the LDO circuit, there are various problems with these configurations. First, the arbitrary voltage level may be set by a voltage that can experience drift as the operating conditions, such as temperature, change. Furthermore, the power amplification circuit may present a load impedance mismatch to the LDO circuit. This in turn, can cause the saturation voltage level of the LDO circuit to change. These short-comings can cause power inefficiencies and/or cause the LDO circuit to be driven into saturation, thereby, resulting in unwanted spectral splatter.
Therefore, what is needed are power amplification devices with voltage regulation circuits designed to reduce power inefficiencies and spectral splatter.